Method of anisotropically etching silicon

ABSTRACT

A method of anisotropic plasma etching of silicon to provide laterally defined recess structures therein through an etching mask employing a plasma, the method including anisotropic plasma etching in an etching step a surface of the silicon by contact with a reactive etching gas to removed material from the surface of the silicon and provide exposed surfaces; polymerizing in a polymerizing step at least one polymer former contained in the plasma onto the surface of the silicon during which the surfaces that were exposed in a preceding etching step are covered by a polymer layer thereby forming a temporary etching stop; and alternatingly repeating the etching step and the polymerizing step. The method provides a high mask selectivity simultaneous with a very high anisotropy of the etched structures.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of anisotropically etching structurespreferably defined with an etching mask, particularly laterally exactlydefined recesses in silicon by means of a plasma.

2. Description of the Related Art

It is known to anisotropically etch defined structures, for exampletrenches, crests, tongues, flexible ridges or the like having low toaverage selectivity, into silicon substrates.

The individual structures to be etched in are usually defined by theetching masks applied to the silicon substrate by way of so-calledmasking layers, for example, a photoresist layer.

In the anisotropic etching technique, it is necessary to achieve alaterally exactly defined recess in the silicon. These deeply-extendingrecesses must have lateral ends which are as exactly vertical aspossible. The edges of the masking layers covering those siliconsubstrate regions that are not supposed to be etched are not underetchedin order to keep the lateral precision of the structural transition fromthe mask into the silicon as high as possible. As a result, it isnecessary to allow the etching to progress only on the bottom of thestructures, but not on the already produced side walls of thestructures.

To this end, it has already been proposed to use a plasma-etching methodto perform etching of profiles in silicon substrates. In this methodchemically reactive species and electrically-charged particles (ions)are generated in a reactive gas mixture in a reactor with the aid of anelectric discharge. The positively-charged cations generated in thismanner are accelerated toward the substrate by means of an electricalprestress applied to the silicon substrate, and fall virtuallyvertically onto the substrate surface, and promote the chemical reactionof the reactive plasma species with the silicon on the etching base.

Because of the nearly vertical fall of the cations, etching shouldprogress correspondingly slowly toward the side walls of the structures,e.g., in the optimum case, not at all.

It is known to use non-dangerous and process-stable reactive gases basedon fluorochemicals. However, in this case it is very disadvantageousthat these reactive gases acting on a fluorochemical basis permit a veryhigh etching rate and a very high selectivity, but display a markedlyisotropic etching behavior.

In comparison to the silicon, the fluorine radicals generated in theplasma have such a high spontaneous reaction rate that the structureedges (lateral surfaces) are etched quickly, thus resulting in undesiredunderetching of the mask edges.

Moreover, it has already been proposed to cover the side walls withpolymer formers which are located in the plasma at the same time duringetching, and to protect the walls by means of this polymer film. Becausethis polymer film would also form on the etching base, a stable fall ofions should keep this film free from polymer and permit etching there.However, associated with this is the disadvantage that the polymerformers added to the plasma, which are partly formed from the fluorinecarrier itself or through the splitting of fluorine radicals, or whichresult from purposely added, unsaturated compounds or eroded, organicmask material photoresist), occur as recombination partners with respectto the fluorine radicals. By means of this back reaction, the objectiveof which is a chemical equilibrium, a considerable portion of thefluorine required for etching is neutralized, while at the same time acorresponding component of the polymer formers required for side wallpassivation is lost. Because of this, the etching rate that can beattained with this method is markedly reduced.

This dependence of the etching fluorine radicals on the unsaturatedpolymer formers in the plasma makes the etching rates and the etchingprofiles dependent on the free silicon substrate surface to be etched.Furthermore, it is disadvantageous that the unsaturated species presentin the plasma that result in the polymer formers preferably etch certainmask materials and thus cause the selectivity, that is, the ratio of thesilicon etching rate to the mask etching rate, to worsen. Furthermore,if a non-uniform side wall protection occurs, the side walls arepreferably coated directly at the mask edge with polymer, and thus theside wall is better protected in this area than in the progressiveetching depth of the structures.

In this instance the polymer covering of the side walls decreasesrapidly at greater depths, and an underetching occurs there with theconsequence that bottle-type etching profiles result.

Instead of using reactive gases based on fluorine, it has already beenproposed to use reactive gases based on other halogens, particularlychlorine and bromine, which have less avidity, or reactive gases thatrelease chlorine or bromine in plasma.

Because their radicals formed in plasma exhibit a significantly lowerspontaneous reaction with silicon, and first lead to etching withsimultaneous ion support, these reactive gases offer the advantage thatthey essentially etch only on the bottom of the structure, and not onthe side walls of the structure, because the ions impact virtuallyvertically on the silicon substrate. The disadvantage exists, however,that these reactive Oases react in an extraordinarily sensitive mannerwith respect to moisture.

In this case, not only are costly transfer devices necessary for thesilicon substrates in the reactor, but also the leakage rate of theentire etching system must be kept extremely low. Even the slightestoccurrence of reactor moisture leads to microroughness on the bottom ofthe silicon etching due to local silicon oxidation, and thus to acomplete breakdown in etching.

The object of the invention is to create a method of the generic typewith which a high anisotropic etching of silicon substrate can beachieved with fluorochemicals with simultaneous high selectivity.

SUMMARY OF THE INVENTION

According to the invention, the object is accomplished in that theanisotropic etching process is performed separately in separate,respectively alternatingly sequential etching and polymerization steps.

As a consequence of performing anisotropic etching in separate,respectively alternating sequential etching and polymerization steps, inan advantageous manner the simultaneous presence of etching species andpolymer formers in the plasma is avoided altogether. Hence, deepstructures having vertical edges can be realized with very high etchingrates in silicon substrates.

Further advantageous embodiments of the invention ensue from thefeatures disclosed in the dependent claims.

By means of the method of the invention, during the etching step noconsideration need be given to a specific ratio of saturated tounsaturated species, that is, of fluorine radicals to polymer formers,so that the actual etching step can be optimized with respect to theetching rate and selectivity without the anisotropy of the total processbeing adversely affected.

In an advantageous embodiment of the invention, the silicon substratesare bombarded with ionic energy during the etching steps and,alternatively, also during the polymerization steps. By means of thissimultaneous bombardment with ionic energy, it is advantageouslyaccomplished that no polymer can form on the etching base, so that ahigher etching rate can be achieved during the etching step, because aprior, necessary decomposition of the polymer layer on the etching baseis no longer required.

It has been shown that a very good anisotropic result can be achievedwith extraordinarily low ionic energy. As a consequence of the only lowrequired ionic energy, an outstanding mask selectivity can be achieved.

Because the high etching rates that are possible through the method ofthe invention lead to a strongly exothermic reaction of fluorineradicals with silicon, considerable warming of the silicon substrate canensue.

In an advantageous manner, the silicon substrate is cooled during theetching process, preferably by a helium gas current. By means of thesimultaneous cooling of the silicon substrate during the etchingprocess, the advantages of the method of the invention, namely a veryhigh etching rate with simultaneously high selectivity, can be fullyutilized.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described in detail below in conjunction with a drawingwhich schematically shows the design of an etching device that can beused for the method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The FIGURE shows an etching chamber 10, in which a substrate electrode12 connected to a high-frequency supply 14 is disposed.

Furthermore, a Surfatron 16 projects into the etching chamber 10. Asilicon substrate 18 is disposed on the substrate electrode 12 in theworking region of the Surfatron 16. The Surfatron 16 is coupled with aresonator 20 for microwave plasma stimulation. The system further has awaveguide for feeding a reactive gas.

The method of the invention of anisotropically etching silicon substrateis performed in the following manner.

For the sake of an overview, individual references to the etchingchamber 10, in which the method steps take place, are omitted in thefollowing method description.

The etching chamber 10 is also only selected by way of example, and theinvention does not relate in detail to the concrete structure of theetching chamber 10. The method of the invention can, of course, beperformed with an analogous device that completes the individual methodsteps.

A correspondingly prepared silicon substrate, that is, a siliconsubstrate coated with an etching mask, for example of photoresist, withthe etching mask leaving free the regions of the silicon substrate thatare intended to be anisotropically etched, is subjected to a firstetching step.

For this purpose a mixture of, for example, sulfur hexafluoride SF₂ andargon Ar, is used, which has a gas flow of between 0 and 100 sccm and aprocessing pressure between 10 and 100 μbar. In this instance, theplasma generation preferably takes place with a microwave irradiation atoutputs between 300 and 1200W (2.45 GHz).

At the same time, a substrate prestress for ion acceleration is appliedto the substrate electrode. The substrate prestress is preferablybetween 5 and 30V, and can be achieved with a high-frequency supply(13.56 MHz) at outputs between 2 and 10W.

During the etching step, chemically reactive species andelectrically-charged particles (ions) are generated in the reactor--hereSurfatron--with the aid of an electrical discharge in the mixture ofsulfur hexafluoride and argon.

The positively-charged cations generated in this way are acceleratedtoward the silicon substrate by means of the electric prestress appliedto the substrate electrode, and fall nearly vertically onto thesubstrate surface left free by the etching mask, and support thechemical reaction of the reactive plasma species with the silicon.

The etching step can be performed so long that, for example, an etchingdepth of approximately 2-3 μm depth is achieved.

Subsequently, a first polymerization step is performed with a mix of,for example, trifluoromethane CHF₃ and argon Ar. The mixture in thisinstance preferably has a gas flow of from 0 to 100 sccm and a processpressure between 10 and 100 μbar. At an output preferably between 300and 1200W, a microwave irradiation and thus a plasma are generated bymeans of the resonator.

During the polymerization step, the surfaces exposed during the previousetching step, that is, the etching base and the side surfaces, arecovered with a polymer. This polymer layer forms a very effectiveprovisional etching stop on the etching edges or etching surfaces.

The polymer respectively applied to the etching edge in thepolymerization step is partially re-stripped during the second etchingstep which now follows. During the etching step, the edge exposed duringfurther etching already locally experiences an effective protection, bymeans of the polymer partially stripped from the edge region locatedopposite, from a further etching attack.

In the method of the invention, the known tendency of released monomersto precipitate so as to be directly contiguous to one another again hasthe positive consequence of effecting an additional local edgeprotection during further etching. The result of this is that theanisotropy of the individual etching steps, which are effectedseparately from the polymerization steps in the plasma, is significantlyincreased by this effect.

The polymer layer applied to the etching base during the polymerizationstep is rapidly broken through during the subsequent etching step,because the polymer is stripped very quickly with the ion support, andthe chemical reaction of the reactive plasma species with the siliconcan progress on the etching base.

During the etching step, the side walls of the structures to be etchedin remain protected by the polymer applied during the polymerizationstep.

The etching steps and the polymer steps are repeated alternatingly untilthe predetermined etching depth of the structures in the siliconsubstrate is reached. In the microwave-supported method, which permitsan etching rate of between 2 and 20 μm/min, the duration of theindividual etching steps remains such that, for example, 2 to 3 μm depthis further etched per etching step.

The following polymerization step is selected to be approximately solong that, during the polymerization time, an approximately 50 nm thick,Teflon-like polymer layer has precipitated on the side walls or on theetching base. A time of, for example, one minute is required for this.

In an advantageous embodiment of the polymerization step, an ioniceffect on the silicon substrate is performed simultaneously with theapplication of the polymer. For this the substrate electrode is actedupon by a high-frequency output of, for example, 3 to 5W, which resultsin a substrate prestress of approximately 5V. Because the polymer layersthat precipitated during the polymerization step without the ioniceffect are only very slowly etched--only a few nanometers perminute--during the etching step, the simultaneous ion effect during theetching step offers the advantage that the polymer etching rate can bedrastically increased to over 100 nm/min. This is even achieved when thesilicon substrate is bombarded with a low ionic energy, e.g., 5 eV.

If the silicon substrate is already bombarded with low ionic energyduring the polymerization steps, absolutely no polymer can be formed onthe etching base. The monomers capable of polymerization thereforepreferably build up on the side walls, and provide a particularlyeffective protection there against the subsequent etching step, whereasthe etching base remains free from any covering.

During the subsequent etching step, therefore, further etching can takeplace on the etching base without delay, that is, without priorstripping of a polymer film.

With the two alternatives, that is, ionic effect only during the etchingphase or the ionic effect during the etching phase or ion effect duringthe polymerization phase, structure having very high anisotropy, thatis, having practically exactly vertical edge profiles, can be attained.

It is a particular preference that an anisotropic result can be achievedwith extraordinarily low ionic energies. Should no polymer be depositedon the etching base during the polymerization step, ionic energies ofonly approximately 5 eV are sufficient. During the etching steps, an ionbombardment with energies between 5 and 30 eV are recommended in orderto leave the structure base completely free from deposits from theplasma, so that no roughness of the etching base can be established atfirst.

If ions are only accelerated toward the silicon substrate during theetching steps, these are also sufficient to break through, within a fewseconds, the etching base polymer that deposits during thepolymerization steps. In this operating mode, the microloading effect inthe etching rate is further reduced.

Due to the high spontaneous reaction rate of fluorine radicals withsilicon, the silicon etching per se requires no ion support.

A further, essential advantage ensues from the fact that an outstandingmask selectivity is achieved as a consequence of the only low requiredionic energies. Ionic energies of the disclosed magnitude are notsufficient to induce the etching of the mask materials, e.g.,photoresist and silicon oxide, SiO₂, because the activation energy forbreaking chemical bonds in the high-grade cross-linked mask polymer isconsiderably higher. Without a prior breaking of these bonds, it is notpossible for the etching species to react with the mask materials toform volatile compounds which can subsequently be desorbed.

Because high etching rates can be achieved with the described method, awarming of the silicon substrate comes about by means of the stronglyexothermic chemical reaction of fluorine radicals with silicon. Atcorrespondingly high temperatures, the polymers deposited during thepolymerization steps, or also the mask materials, e.g., photoresist,lose their resistance to the etching species. Therefore, it is necessaryto assure sufficient cooling of the silicon substrates. This isaccomplished with methods known per se, e.g., the cooling of the rearside of the silicon substrate by means of a helium gas current, oradhesion of the silicon substrates onto cooled silicon electrodes.

Instead of the described mixtures of sulfur hexafluoride and argon forthe etching steps, or of trifluoromethane and argon for thepolymerization steps, other common etching gases that release fluorine,for example, nitrogen trifluoride NF₃, tetrafluoromethane CF₄ or thelike, can be used just as well for the etching steps, and mixtures basedon perfluorinated aromatic substances having suitable peripheral groups,for example, perfluorinated, styrene-like monomers or ether-likefluorine compounds are used for the polymerization steps.

In all media used, the single crucial point is achieving high densitiesof reactive species and ions with simultaneous low, but preciselycontrollable, energy, with which the generated ions reach thesubstrates.

The ionic energy must be kept as low as possible with consideration fora high mask selectivity. High ionic energies would additionally lead tointerfering reactions of material that dispersed or was stripped andredeposited uncontrolled. The energy of the ions acting on the siliconsubstrate must be sufficient, however, to keep the structure base freefrom deposits, so that a smooth etching base can be obtained.

What is claimed is:
 1. A method of anisotropic plasma etching of silicon to provide laterally defined recess structures therein through an etching mask employing a plasma, the method comprising:a. anisotropic plasma etching in an etching step a surface of the silicon by contact with a reactive etching gas to removed material from the surface of the silicon and provide exposed surfaces; b. polymerizing in a polymerizing step at least one polymer former contained in the plasma onto the surface of the silicon during which the surfaces that were exposed in a preceding etching step are covered by a polymer layer thereby forming a temporary etching stop; and c. alternatingly repeating the etching step and the polymerizing step.
 2. The method according to claim 1, wherein the polymerization steps are controlled independently of one another.
 3. The method according to claim 1, wherein the etching steps are performed without the presence of polymer formers in the plasma.
 4. The method according to claim 1, wherein a polymer layer applied during the polymerization step to the laterally defined recess structures is partially restripped during an immediately subsequent etching step.
 5. The method according to claim 1, wherein etching removes material from the surface of the silicon to a preselected etching depth, and wherein the etching steps are performed over a preselected period of time which provides the preselected etching depth.
 6. The method according to claim 1, wherein polymerizing polymer formers contained in the plasma onto the surface of the silicon provides a polymer layer having a preselected thickness, and wherein the polymerization steps are performed over a preselected period of time which provides the preselected thickness.
 7. The method according to claim 1, wherein the silicon is bombarded by an ionic energy during the etching steps.
 8. The method according to claim 1, wherein the silicon is alternatingly bombarded by an ionic energy during the polymerization steps.
 9. The method according to claim 1, wherein the silicon is bombarded by an ionic energy during the etching steps, and wherein the ionic energy ranges between 1 and 50 eV during the etching steps.
 10. The method according to claim 9, wherein the ionic energy ranges between 5 and 30 eV during the etching steps.
 11. The method according to claim 1, wherein the silicon is bombarded by an ionic energy during the polymerization steps, and wherein, during the polymerization steps, the ionic energy ranges between 1 and 10 eV.
 12. The method according to claim 11, wherein the ionic energy ranges between 4 and 6 eV.
 13. The method according to claim 12, wherein the ionic energy is 5 eV.
 14. The method according to claim 1, wherein the reactive etching gas is at least one gas that releases fluorine.
 15. The method according to claim 1, wherein the reactive etching gas is a mixture of sulfur hexafluoride, SF₆, and argon, Ar.
 16. The method according to claim 1, wherein the at least one polymer former contained in the plasma is at least one hydrofluorocarbon.
 17. The method according to claim 16, wherein the at least one hydrofluorocarbon has a low fluorine-to-carbon ratio.
 18. The method according to claim 1, wherein the at least one polymer former contained in the plasma is a mixture of trifluoromethane, CHF₃, and argon, Ar.
 19. The method according to claim 1, wherein the etching steps and polymerization steps employ the recited respective media have gas flows ranging from 0 to 100 sccm and processing pressures ranging from 10 to 100 μbar.
 20. The method according to claim 1, wherein the plasma is generated with microwave irradiation at an output ranging between 100 and 1500 W.
 21. The method according to claim 1, wherein the plasma is generated with microwave irradiation at an output ranging between 300 and 1200 W.
 22. The method according to claim 1, wherein the silicon is cooled during at least one of the etching steps and the polymerization steps.
 23. The method according to claim 1, wherein the silicon has a rear side, and wherein the rear side of the silicon is acted upon by a helium gas current.
 24. The method according to claim 1, wherein a cooled substrate electrode is provided with a thermal contact material, and wherein the silicon is positioned on the thermal contact material.
 25. The method according to claim 1, wherein the etching steps and polymerization steps are preformed with a high plasma density of reactive species and ions.
 26. The method according to claim 1, wherein the plasma density and the ionic energy are controlled independently of one another. 